From Bugzilla Helper:
User-Agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.3b) Gecko/20030211
Description of problem:
Pressing Ctrl+Alt+Backspace doesn't kill the X server. This has been observed
on three different system running Phoebe-3.
What is really odd is that Ctrl+Alt+Fx to switch VT works on exactly one of
these three systems, on the other two it is broken as well.
Version-Release number of selected component (if applicable):
Free86-4.2.99.3-20030115.0
How reproducible:
Always
Steps to Reproduce:
1. Log in using the graphical login in a KDE session
2. Press Ctrl+Alt+Backspace to zap the X server
3.
Actual Results: Nothing
Expected Results: The X server should have died and the login screen reappeared
Additional info:

There were some 'issues' with xkb all along the latter months of XFree86
4.2.99.x, but all known issues were fixed prior to the final release.
Please test 4.3.0-2.1 from ftp://people.redhat.com/mharris/testing/unstable
and update me if it fixes it for ya.

I can kill the X server using CTRL-ALT-BS in a fresh Red Hat Linux install
on an Intel Northwood system with an i865 chipset. I believe the problem
was fixed between Phoebe beta 3 and the final release.
If the problem still occurs for you with RHL 9, please reopen.
Thanks.

I have identified that the problem is with the Xmodmap file -- unlike previous
versions of XFree86, the current XFree86 seems to require explicit Xmodmap
entries for the zap functions. It claims they are synthesized if not present,
but that doesn't actually seem to be the case.
Adding the relevant XF86_* entries to the Xmodmap file resolved the issue.